K. Popper - Logic scientific discovery
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K. Popper - Logic scientific discovery

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For they alone could overthrow sentences—sentences
other than protocol sentences, of course. But if they are deprived of this
function, and if they themselves can be overthrown by theories, what
are they for? Since Neurath does not try to solve the problem of demar-
cation, it seems that his idea of protocol sentences is merely a relic—a
surviving memorial of the traditional view that empirical science starts
from perception.


I propose to look at science in a way which is slightly different
from that favoured by the various psychologistic schools: I wish to
distinguish sharply between objective science on the one hand, and ‘our knowledge’ on the

I readily admit that only observation can give us ‘knowledge con-
cerning facts’, and that we can (as Hahn says) ‘become aware of facts
only by observation’.1 But this awareness, this knowledge of ours,
does not justify or establish the truth of any statement. I do not believe,
therefore, that the question which epistemology must ask is, ‘. . . on
what does our knowledge rest? . . . or more exactly, how can I, having had
the experience S. justify my description of it, and defend it against
doubt?’2 This will not do, even if we change the term ‘experience’ into
‘protocol sentence’. In my view, what epistemology has to ask is,
rather: how do we test scientific statements by their deductive

1 H. Hahn, Logik, Mathematik und Naturerkennen, in Einheitswissenschaft 2, 1933, pp. 19 and 24.
2 Cf. Carnap, for instance, Scheinprobleme in der Philosophie, 1928, p. 15 (no italics in the

the problem of the empirical basis 79

consequences?*1 And what kind of consequences can we select for this
purpose if they in their turn are to be inter-subjectively testable?

By now, this kind of objective and non-psychological approach is
pretty generally accepted where logical or tautological statements are
concerned. Yet not so long ago it was held that logic was a science
dealing with mental processes and their laws—the laws of our thought.
On this view there was no other justification to be found for logic than
the alleged fact that we just could not think in any other way. A logical
inference seemed to be justified because it was experienced as a neces-
sity of thought, as a feeling of being compelled to think along certain
lines. In the field of logic, this kind of psychologism is now perhaps a
thing of the past. Nobody would dream of justifying the validity of a
logical inference, or of defending it against doubts, by writing beside it
in the margin the following protocol sentence. ‘Protocol: In checking
this chain of inferences today, I experienced an acute feeling of

The position is very different when we come to empirical statements of
science. Here everybody believes that these are grounded on experiences
such as perceptions; or in the formal mode of speech, on protocol
sentences. Most people would see that any attempt to base logical
statements on protocol sentences is a case of psychologism. But curi-
ously enough, when it comes to empirical statements, the same kind of
thing goes today by the name of ‘physicalism’. Yet whether statements
of logic are in question or statements of empirical science, I think the
answer is the same: our knowledge, which may be described vaguely as a
system of dispositions, and which may be of concern to psychology, may
be in both cases linked with feelings of belief or of conviction: in the
one case, perhaps, with the feeling of being compelled to think in a
certain way; in the other with that of ‘perceptual assurance’. But all this
interests only the psychologist. It does not even touch upon problems
like those of the logical connections between scientific statements,
which alone interest the epistemologist.

*1 At present, I should formulate this question thus. How can we best criticize our theories
(our hypotheses, our guesses), rather than defend them against doubt? Of course, testing
was always, in my view, part of criticizing. (Cf. my Postscript, sections *7, text between notes
5 and 6, and end of *52.)

some structural components of a theory of experience80

(There is a widespread belief that the statement ‘I see that this table
here is white’, possesses some profound advantage over the statement
‘This table here is white’, from the point of view of epistemology. But
from the point of view of evaluating its possible objective tests, the first
statement, in speaking about me, does not appear more secure than the
second statement, which speaks about the table here.)

There is only one way to make sure of the validity of a chain of
logical reasoning. This is to put it in the form in which it is most easily
testable: we break it up into many small steps, each easy to check by
anybody who has learnt the mathematical or logical technique of trans-
forming sentences. If after this anybody still raises doubts then we can
only beg him to point out an error in the steps of the proof, or to think
the matter over again. In the case of the empirical sciences, the situation
is much the same. Any empirical scientific statement can be presented
(by describing experimental arrangements, etc.) in such a way that
anyone who has learned the relevant technique can test it. If, as a result,
he rejects the statement, then it will not satisfy us if he tells us all about
his feelings of doubt or about his feelings of conviction as to his
perceptions. What he must do is to formulate an assertion which con-
tradicts our own, and give us his instructions for testing it. If he fails to
do this we can only ask him to take another and perhaps a more careful
look at our experiment, and think again.

An assertion which owing to its logical form is not testable can at
best operate, within science, as stimulus: it can suggest a problem. In
the field of logic and mathematics, this may be exemplified by Fermat’s
problem, and in the field of natural history, say, by reports about sea-
serpents. In such cases science does not say that the reports are
unfounded; that Fermat was in error or that all the records of observed
sea-serpents are lies. Instead, it suspends judgment.3

Science can be viewed from various standpoints, not only from that
of epistemology; for example, we can look at it as a biological or as a
sociological phenomenon. As such it might be described as a tool, or
an instrument, comparable perhaps to some of our industrial
machinery. Science may be regarded as a means of production—as the

3 Cf. the remark on ‘occult effects’ in section 8.

the problem of the empirical basis 81

last word in ‘roundabout production’.4 Even from this point of view
science is no more closely connected with ‘our experience’ than other
instruments or means of production. And even if we look at it as
gratifying our intellectual needs, its connection with our experiences
does not differ in principle from that of any other objective structure.
Admittedly it is not incorrect to say that science is ‘. . . an instrument’
whose purpose is ‘. . . to predict from immediate or given experiences
later experiences, and even as far as possible to control them’.5 But I do
not think that this talk about experiences contributes to clarity. It has
hardly more point than, say, the not incorrect characterization of an oil
derrick by the assertion that its purpose is to give us certain experi-
ences: not oil, but rather the sight and smell of oil; not money, but
rather the feeling of having money.


It has already been briefly indicated what rôle the basic statements play
within the epistemological theory I advocate. We need them in order to
decide whether a theory is to be called falsifiable, i.e. empirical. (Cf.
section 21.) And we also need them for the corroboration of falsifying
hypotheses, and thus for the falsification of theories. (Cf. section 22.)

Basic statements